The 455 was not offered by Oldsmobile until the 1968 model year. As has been said, if your car really is original, it would have a 425.
There's good information here about identifying big block engines of that era:
I have a '67 Delta 88 with the 425, and it has the casting code shown next to the letter "D" in the table of information in the above link. Specifically, mine has the code 389244D, with the "D" about twice the size as the numbers.
Go take a look at your engine, and I'll bet it has the same number. It should be on a little shelf right under the little coolant hose that comes out of the thermostat housing and enters the top of the water pump.